Role of orientation relationship for the formation of morphology and preferred orientation in NiAl-(Cr,Mo) during directional solidification

In order to reveal the role of orientation relationship for the formation of morphology and preferred orientation in directionally solidified NiAl-(Cr,Mo) eutectic alloys, a unique in-situ X-ray diffraction setup was utilized. Comprehensive analyses of the solidification front during processing demonstrate that the eutectic alloys crystallize with preferred crystallographic orientation of both phases with respect to the growth direction. These orientations of the eutectics prevail during growth of the fibrous or lamellar colonies and subsequent cooling as determined by ex-situ microstructural analysis. Depending on the composition and growth velocity, different preferred orientations were obtained resolving the ambiguity in literature regarding the reported components, e.g. strong 〈001〉 and 〈111〉 directions parallel to the growth direction but also additional minor 〈110〉 components. By the aid of in-situ experiments excluding the superimposed effect of different thermal expansion of the phases in comparison to previous experiments, a correlation between the evolving preferred orientation and lattice mismatch during solidification was found for alloys with low Mo content. Remarkably, no change in morphology between fibers and lamellae was associated to the transitions in preferred orientation. For larger Mo contents, two distinct orientation relationships between the A2 (Cr,Mo) solid solution and the B2 NiAl intermetallic phase were identified by ex-situ microstructural analysis, namely (1) cube-on-cube and (2) a 60° rotation about 〈111〉 in different colonies of one and the same sample. The morphology of these colonies were fibrous and lamellar, respectively. For the 60° relationship, the growth direction of both phase is unique, namely 〈111〉. Thus, morphology and preferred orientation are determined by the orientation relationship in this case.